A known position sensor compares the output of a sensor means with a given threshold level to detect the position of an object whose position should be detected, to control the position of the object, and to perform a timing control operation. We now describe an apparatus for detecting the position of a shutter blade included in a shutter mechanism as an example. Generally, immediately after the opening formed by shutter blades is released, the operating characteristics of the shutter blades are not stable by the effects of the camera's posture and for other causes. In order to obtain an adequate exposure time, a certain preparatory interval is required until the operating characteristics of the shutter blades become stable. Accordingly, in an ordinary shutter mechanism, after the opening formed by the shutter blades is released, the blades go through the preparatory interval and move until a pinhole is formed. This state is detected. In response to this, the exposure time and the timing at which the strobe is synchronized are controlled.
Good examples of this position sensor are widespread position sensors using a light switch such as a photo-interrupter or photo-reflector. In such a widespread position sensor, shutter blades are moved so as to block and unblock an optical path going from a light-emitting portion to a light-receiving portion. FIG. 6 illustrates the principle of one example of a blade position detector using a photo-reflector. In the illustrated circuit, a reflector 3 is disposed in an optical path going from a light-emitting diode 1 to a phototransistor 2. The front end, for example, of a shutter blade 4 is made to pass through the optical path containing the reflector 3. The shutter blade 4 blocks the optical path and then releases it. At this instant, the phototransistor 2 is turned on. This turns on a switching transistor 5. At this instant, an exposure control circuit 6 is started to be triggered.
However, the output from the light-emitting diode, the reflectivity of the photo-reflector, the sensitivity of the phototransistor, and other factors vary among different commercial products. Therefore, where the above-described method is employed, one conventional technique is to adjust the output from the light-emitting diode 1 by means of a variable resistor 7 or the like installed in a circuit when the apparatus is shipped. However, the camera user is not urged to periodically inspect the camera with certainty. Consequently, this method cannot cope with a situation in which the camera ages and the performance of each component of the photo-reflector changes. Another problem with a camera is that its power source depends on a dry battery or dry cell. That is, the stability of the power source is quite low. Hence, an accurate voltage-regulated circuit is necessitated.